Soil samples for volatile organic compound (VOC) analysis are usually shipped to the laboratory in coolers with ice packs at refrigerator temperatures near 4° C. Once they arrive at the laboratory, the samples are either kept in a refrigerator prior to analysis, or they are placed in a freezer for longer-term storage. Both The U.S. Environmental Protection Agency (EPA) and ASTM International (ASTM) recognize the benefit of freezing samples, and many discussions have taken place with these entities concerning the feasibility of freezing samples in the field and shipping them at freezer temperatures (<−7 to −20° C.) to the laboratory for analysis. Using freezing as a preservation technique, sample holding time can be extended from 48 hours to 14 days.
Two possible ways of shipping frozen samples are to use a small, power-operated freezer compartment or dry ice storage. However, use of a power-operated freezer in the field is not feasible in most cases (indeed, in preferred embodiments of the inventive technology, no cooling is provided by electrical power). Dry ice storage is also not a viable option since air shipment of packages containing dry ice is regulated, because dry ice sublimes to gaseous carbon dioxide, which can displace air in sealed aircraft. Even more important relative to sample storage is that dry ice has a temperature of −78° C., which is so cold that it will cause the seals of sample containers to be compromised, and VOCs will be lost from samples when they thaw. Certain embodiments of the inventive technology disclosed and claimed herein seek to alleviate one or more of such problems.
ASTM and EPA are prescribing freezing temperatures of approximately −12±5° C. for storage of soil samples containing VOCs during shipment to the laboratory for analysis (ASTM 2007a, 2007b, U.S. EPA 2002). Phase change materials (PCMs) have the property of storing or releasing heat energy at a specific temperature, which is the temperature of fusion. For example, water acts as a phase change material (PCM) at 0° C. Salt-water solutions melt at lower temperatures than water depending on the salt type and concentration. Sodium chloride (NaCl) solutions are used to achieve temperatures of fusion below 0° C. A water/urea phase change formulation that has a melting range of −11 to −15° C. has previously been described (Salyer 1997). Different PCM formulations can be used depending on the needs and constraints (e.g., required sample temperature range and length of shipping or storage time) of the application. For example, in those applications where only refrigerator temps (e.g., 34-40 degrees F.) are needed, perhaps a pure water PCM formulation is adequate; in others, saltwater may be used. PCM formulations may be, but certainly are not limited to, those described in U.S. Pat. No. 7,260,956, or U.S. Pat. No. 7,516,600, as but two examples, and those particularly described in this disclosure.
In addition to the temperature of fusion, the heat of fusion is an important parameter. The higher the heat of fusion, the greater the capacity for the material to store or release energy at the temperature of fusion. The heat of fusion of water is near 80 cal/g (Bolz and Tuve 1980). A PCM formulation to be used in a cooler for shipping frozen samples should have as high a heat of fusion as possible. Addition of chemicals to water can lower the freezing point, but can also decrease the heat of fusion. Therefore, an optimal PCM formulation has a temperature of fusion in the desired temperature range while having a heat of fusion as close to that of water as possible.